Glucose homeostasis and the gastrointestinal tract: insights into the treatment of diabetes

The gastrointestinal tract is increasingly viewed as a critical organ in glucose metabolism because of its role in delivering glucose to the circulation and in secreting multiple glucoregulatory hormones that, in concert with insulin and glucagon, regulate glucose homeostasis. Under normal conditions, a complex interplay of these hormones acts to maintain plasma glucose within a narrow range despite large variations in the availability of glucose, particularly during transition from the fasting to fed state. In the fed state, the rate at which nutrients are passed from the stomach to the duodenum, termed gastric emptying rate, is a key determinant of postprandial glucose flux. In patients with diabetes, the regulation of glucose metabolism is disrupted resulting in fasting and postprandial hyperglycaemia. Elucidation of the role of the gastrointestinal tract, gut-derived glucoregulatory peptides and gastric emptying rate offers a new perspective on glucose homeostasis and the respective importance of these factors in the diabetes state. This review will highlight the importance of the gastrointestinal tract in playing a key role in glucose homeostasis, particularly in the postprandial period, and the role of established or new therapies that either leverage or modify gastrointestinal function to improve glycaemic state.

Preserved GLP-1 effects in a diabetic patient with Cushing's disease

CONTEXT

A patient with diabetes mellitus, who participated in a study with intravenous administration of GLP-1, was later found to have Cushing's disease (markedly elevated 24 h urinary cortisol excretion and inadequate suppression of fasting cortisol with 2 mg dexamethasone). His diabetic state disappeared (2 h plasma glucose after 75 g oral glucose 159 mg/dl=IGT) after successful pituitary surgery (normal 24 h urinary cortisol excretion and adequate cortisol suppression with 2 mg dexamethasone).

OBJECTIVE

The present analysis was undertaken to compare GLP-1 actions on fasting glycemia in diabetes mellitus due to Cushing's disease with GLP-1 actions in typical type 2 diabetes.

DESIGN AND METHODS

GLP-1 (1.2 pmol/kg/min) and placebo had been infused into ten patients with diabetes mellitus over 4 h in the fasting state. The results from the patient with Cushing's disease (C) were compared to the data from the remaining nine patients with type 2 diabetes (D).

RESULTS

Within 4 h glucose decreased from basal (C: 12.9; D: 12.9+/-0.7 mmol/l) to normal fasting values (C: 5.0; D: 4.9+/-0.4 mmol/l). The stimulation of insulin secretion and suppression of glucagon secretion was similar in the patient with Cushing's disease compared to those with type 2 diabetes.

CONCLUSIONS

The insulinotropic, glucagonostatic and glucose-lowering actions of GLP-1 in a patient with diabetes mellitus due to cortisol excess were similar to actions in typical type 2 diabetes. Therefore incretin mimetics might be a novel therapeutic strategy for the treatment of glucocorticoid-induced diabetes mellitus.

Suppression of glucagon secretion is lower after oral glucose administration than during intravenous glucose administration in human subjects

AIMS/HYPOTHESIS

The incretin effect describes the augmentation of postprandial insulin secretion by gut hormones. It is not known whether glucagon secretion is also influenced by an incretin effect. A glucagon suppression deficiency has been reported in some patients with type 2 diabetes, but it is unclear whether this abnormality is present prior to diabetes onset. We therefore addressed the questions: (1) Is glucagon secretion different after oral and during intravenous glucose administration? (2) If so, is this related to the secretion of incretin hormones? (3) Is glucagon secretion abnormal in first-degree relatives of patients with type 2 diabetes?

MATERIALS AND METHODS

We examined 16 first-degree relatives of patients with type 2 diabetes and ten matched control subjects with an oral glucose load (75 g) and with an 'isoglycaemic' intravenous glucose infusion.

RESULTS

Glucagon levels were significantly suppressed by both oral and intravenous glucose (p < 0.0001), but glucagon suppression was more pronounced during intravenous glucose administration (76 +/- 2%) than after oral glucose administration (48 +/- 4%; p < 0.001). The differences in the glucagon responses to oral and i.v. glucose were correlated with the increments in gastric inhibitory polypeptide (GIP) (r = 0.60, p = 0.001) and glucagon-like peptide (GLP)-1 (r = 0.46, p < 0.05). There were no differences in glucagon levels between first-degree relatives and control subjects.

CONCLUSIONS/INTERPRETATION

Despite the glucagonostatic actions of GLP-1, the suppression of glucagon secretion by glucose is diminished after oral glucose ingestion, possibly due to the glucagonotropic actions of GIP and GLP-2. Furthermore, in this group of first-degree relatives, abnormalities in glucagon secretion did not precede the development of other defects, such as impaired insulin secretion.

Efficacy and safety of the dipeptidyl peptidase-4 inhibitor, sitagliptin, compared with the sulfonylurea, glipizide, in patients with type 2 diabetes inadequately controlled on metformin alone: a randomized, double-blind, non-inferiority trial

AIM

To compare the efficacy and safety of sitagliptin vs. glipizide in patients with type 2 diabetes and inadequate glycaemic control [haemoglobin A(1c) (HbA(1c)) > or = 6.5 and < or = 10%] on metformin monotherapy.

METHODS

After a metformin dose titration/stabilization period (> or = 1500 mg/day), 1172 patients were randomized to the addition of sitagliptin 100 mg q.d. (N = 588) or glipizide 5 mg/day (uptitrated to a potential maximum 20 mg/day) (N = 584) for 52 weeks. The primary analysis assessed whether sitagliptin was non-inferior to glipizide regarding HbA(1c) changes from baseline at Week 52 using a per-protocol approach.

RESULTS

From a mean baseline of 7.5%, HbA(1c) changes from baseline were -0.67% at Week 52 in both groups, confirming non-inferiority. The proportions achieving an HbA(1c) < 7% were 63% (sitagliptin) and 59% (glipizide). Fasting plasma glucose changes from baseline were -0.56 mmol/l (-10.0 mg/dl) and -0.42 mmol/l (-7.5 mg/dl) for sitagliptin and glipizide, respectively. The proportion of patients experiencing hypoglycaemia episodes was significantly (p < 0.001) higher with glipizide (32%) than with sitagliptin (5%), with 657 events in glipizide-treated patients compared with 50 events in sitagliptin-treated patients. Sitagliptin led to weight loss (change from baseline =-1.5 kg) compared with weight gain (+1.1 kg) with glipizide [between-treatment difference (95% confidence interval) =-2.5 kg (-3.1, -2.0); p < 0.001].

CONCLUSIONS

In this study, the addition of sitagliptin compared with glipizide provided similar HbA(1c)-lowering efficacy over 52 weeks in patients on ongoing metformin therapy. Sitagliptin was generally well tolerated, with a lower risk of hypoglycaemia relative to glipizide and with weight loss compared with weight gain with glipizide.

Efficacy and safety of the dipeptidyl peptidase-4 inhibitor, sitagliptin, compared with the sulfonylurea, glipizide, in patients with type 2 diabetes inadequately controlled on metformin alone: a randomized, double-blind, non-inferiority trial

AIM

To compare the efficacy and safety of sitagliptin vs. glipizide in patients with type 2 diabetes and inadequate glycaemic control [haemoglobin A(1c) (HbA(1c)) > or = 6.5 and < or = 10%] on metformin monotherapy.

METHODS

After a metformin dose titration/stabilization period (> or = 1500 mg/day), 1172 patients were randomized to the addition of sitagliptin 100 mg q.d. (N = 588) or glipizide 5 mg/day (uptitrated to a potential maximum 20 mg/day) (N = 584) for 52 weeks. The primary analysis assessed whether sitagliptin was non-inferior to glipizide regarding HbA(1c) changes from baseline at Week 52 using a per-protocol approach.

RESULTS

From a mean baseline of 7.5%, HbA(1c) changes from baseline were -0.67% at Week 52 in both groups, confirming non-inferiority. The proportions achieving an HbA(1c) < 7% were 63% (sitagliptin) and 59% (glipizide). Fasting plasma glucose changes from baseline were -0.56 mmol/l (-10.0 mg/dl) and -0.42 mmol/l (-7.5 mg/dl) for sitagliptin and glipizide, respectively. The proportion of patients experiencing hypoglycaemia episodes was significantly (p < 0.001) higher with glipizide (32%) than with sitagliptin (5%), with 657 events in glipizide-treated patients compared with 50 events in sitagliptin-treated patients. Sitagliptin led to weight loss (change from baseline =-1.5 kg) compared with weight gain (+1.1 kg) with glipizide [between-treatment difference (95% confidence interval) =-2.5 kg (-3.1, -2.0); p < 0.001].

CONCLUSIONS

In this study, the addition of sitagliptin compared with glipizide provided similar HbA(1c)-lowering efficacy over 52 weeks in patients on ongoing metformin therapy. Sitagliptin was generally well tolerated, with a lower risk of hypoglycaemia relative to glipizide and with weight loss compared with weight gain with glipizide.

A comparison of twice-daily exenatide and biphasic insulin aspart in patients with type 2 diabetes who were suboptimally controlled with sulfonylurea and metformin: a non-inferiority study

AIMS/HYPOTHESIS

The aim of this 52-week, open-label, non-inferiority trial was to compare the safety and efficacy of exenatide (an incretin mimetic) with that of biphasic insulin aspart.

MATERIALS AND METHODS

Patients on metformin and a sulfonylurea were randomised to exenatide (n = 253; 5 microg twice daily for 4 weeks, 10 microg thereafter) or biphasic insulin aspart (n = 248; twice-daily doses titrated for optimal glucose control), while continuing with metformin and sulfonylurea treatment.

RESULTS

Glycaemic control achieved with exenatide was non-inferior to that achieved with biphasic insulin aspart (mean+/-SEM, HbA(1c) change: exenatide -1.04 +/- 0.07%, biphasic insulin aspart -0.89 +/- 0.06%; difference -0.15 [95% CI -0.32 to 0.01]%). Exenatide-treated patients lost weight, while patients treated with biphasic insulin aspart gained weight [between-group difference -5.4 (95% CI -5.9 to -5.0) kg]. Both treatments reduced fasting serum glucose (exenatide -1.8 +/- 0.2 mmol/l, p < 0.001; biphasic insulin aspart -1.7 +/- 0.2 mmol/l, p < 0.001). Greater reductions in postprandial glucose excursions following morning (p < 0.001), midday (p = 0.002) and evening meals (p < 0.001) were observed with exenatide. The withdrawal rate was 21.3% (54/253) for exenatide and 10.1% (25/248) for biphasic insulin aspart. Nausea (33% incidence, 3.5% discontinuation) was the most common adverse event observed with exenatide.

CONCLUSIONS/INTERPRETATION

Exenatide treatment resulted in HbA(1c) reduction similar to biphasic insulin aspart and provided better postprandial glycaemic control, making it a potential alternative for the treatment of type 2 diabetes. Treatment with biphasic insulin aspart was associated with weight gain and lower risk of adverse gastrointestinal events. Although the availability of glucose-lowering agents associated with weight reduction may be considered a therapeutic advance, the long-term implications of progressive weight reduction observed with exenatide have yet to be defined.

Five Weeks of Treatment with the GLP-1 Analogue Liraglutide Improves Glycaemic Control and Lowers Body weight in Subjects with Type 2 Diabetes

AIMS

Effects of the long acting GLP-1 analogue--liraglutide in subjects with type 2 diabetes.

METHODS

144 type 2 diabetic subjects on metformin treatment (1000 mg BID) were randomised to 5 weeks of treatment (double-blind) with metformin plus liraglutide, liraglutide or metformin, or metformin plus glimepiride (open label). The dose of liraglutide was increased weekly from 0.5 to 2 mg OD.

RESULTS

Liraglutide added to metformin monotherapy was associated with a significant reduction in fasting serum glucose (FSG) (-3.9 mM -4.9; -2.9) (primary objective), and HbA1c levels (-0.8% -1.2; -0.4). Furthermore, liraglutide in combination with metformin vs. metformin plus glimepiride significantly reduced FSG (-1.2 mM -2.2; -0.2). In addition, body weight was significantly lower in the metformin plus liraglutide vs. the metformin plus glimepiride group (-2.9 kg -3.6; -2.1). There were no biochemically confirmed episodes of hypoglycaemia with liraglutide treatment. Nausea was the most frequently reported adverse event following liraglutide therapy, it was transient in nature, and led to withdrawal of only 4% of the subjects.

CONCLUSIONS

Using a weekly dose-titration liraglutide is well tolerated up to 2 mg daily. While liraglutide caused transient gastrointestinal side effects, this rarely interfered with continuing treatment. An improvement in FSG over that in control groups was seen for liraglutide as an add-on to metformin. In the latter case, body weight was reduced in comparison to metformin plus glimepiride. Liraglutide is a promising drug for the treatment of type 2 diabetes.

Glucagon-like peptide 1 abolishes the postprandial rise in triglyceride concentrations and lowers levels of non-esterified fatty acids in humans

AIMS/HYPOTHESIS

Diabetic dyslipidaemia contributes to the excess morbidity and mortality in patients with type 2 diabetes. Exogenous glucagon-like peptide 1 (GLP-1) lowers postprandial glycaemia predominantly by slowing gastric emptying. Therefore, the effects of GLP-1 on postprandial lipid levels and gastric emptying were assessed.

METHODS

14 healthy male volunteers were studied with an i.v. infusion of GLP-1 (1.2 pmol kg(-1) min(-1)) or placebo over 390 min in the fasting state. A solid test meal was served and gastric emptying was determined using a (13)C-labelled sodium octanoate breath test. Venous blood was drawn frequently for measurement of glucose, insulin, C-peptide, glucagon, GLP-1, triglycerides and NEFA.

RESULTS

GLP-1 administration lowered fasting and postprandial glycaemia (p<0.0001). Gastric emptying was delayed by GLP-1 compared with placebo (p<0.0001). During GLP-1 administration, insulin secretory responses were higher in the fasting state but lower after meal ingestion. After meal ingestion, triglyceride plasma levels increased by 0.33+/-0.14 mmol/l in the placebo experiments (p<0.0001). In contrast, the postprandial increase in triglyceride levels was completely abolished by GLP-1 (change in triglycerides, -0.023+/-0.045 mmol/l; p<0.05). During GLP-1 infusion, plasma concentrations of NEFA were suppressed by 39% in the fasting state (p<0.01) and by 31+/-5% after meal ingestion (p<0.01).

CONCLUSIONS/INTERPRETATION

GLP-1 improves postprandial lipidaemia, presumably as a result of delayed gastric emptying and insulin-mediated inhibition of lipolysis. Thus, by lowering both glucose and lipid concentrations, GLP-1 administration may reduce the cardiovascular risk in patients with type 2 diabetes.

To be or not to be--an incretin or enterogastrone?

Glucagon-like peptide 1 does not comfortably fulfil the criterion of a gut derived factor responsible for an enhanced meal related insulin response; it appears logical to add the definition of a "physiological incretin hormone".

Secretion of incretin hormones and the insulinotropic effect of gastric inhibitory polypeptide in women with a history of gestational diabetes

AIMS/HYPOTHESIS

The insulinotropic effect of gastric inhibitory polypeptide (GIP) is reduced in patients with type 2 diabetes and around 50% of their first-degree relatives under hyperglycaemic conditions. It is unknown whether this is a result of a specific defect in GIP action or of a general reduction in beta cell function. Moreover, impaired secretion of glucagon-like peptide 1 (GLP-1) has been described in patients with type 2 diabetes. Therefore, we studied the insulinotropic effect of GIP in women with previous gestational diabetes (pGDM) under euglycaemic fasting conditions and during a hyperglycaemic clamp experiment. The secretion of GIP and GLP-1 was assessed following oral glucose ingestion.

MATERIALS AND METHODS

On separate occasions we performed an OGTT and administered an i.v. bolus of 20 pmol GIP/kg body weight in 20 women with pGDM and 20 control women. An additional hyperglycaemic clamp experiment (140 mg/dl [7.8 mmol/l] over 120 min) with i.v. infusion of GIP (2 pmol kg(-1) min(-1); 30-90 min) was performed in 14 women in each group. Capillary and venous blood samples were drawn for the measurement of glucose (glucose oxidase), insulin, C-peptide, GIP and GLP-1 (specific immunoassays). Indices of insulin sensitivity and beta cell function were calculated. Statistical analyses were carried out using repeated measures ANOVA.

RESULTS

Following oral glucose ingestion, plasma glucose, insulin and C-peptide concentrations increased to higher levels in the women with pGDM than in the control women (p<0.05). The women with pGDM were characterised by a higher degree of insulin resistance than the control women (p=0.007 for the Matsuda index), but showed no overt defects in glucose-stimulated insulin secretion (p=0.40 for the insulinogenic index following i.v. glucose). The secretion of GLP-1 and GIP was not different between the groups (p=0.87 and p=0.57, respectively). The insulin secretory response to GIP administration was similar in the two groups both after GIP bolus administration and during the hyperglycaemic clamp experiment (p=0.99 and p=0.88, respectively). A hyperbola-like relationship was found between the degree of insulin sensitivity (Matsuda index) and the insulin secretory response to GIP and i.v. glucose administration.

CONCLUSIONS/INTERPRETATION

These results do not support the hypothesis of an early defect in GIP action as a risk factor for subsequent development of diabetes in women with previous gestational diabetes. The inverse relationship between insulin resistance and the insulin secretory response to glucose or GIP suggests that beta cell secretory function in response to different stimuli increases adaptively when insulin sensitivity is diminished.

alpha-Glucosidase inhibition (acarbose) fails to enhance secretion of glucagon-like peptide 1 (7-36 amide) and to delay gastric emptying in Type 2 diabetic patients

AIM

Acarbose is able to enhance GLP-1 release and delay gastric emptying in normal subjects. The effect of alpha-glucosidase inhibition on GLP-1 has been less evident in Type 2 diabetic patients. The aim of this study was to investigate the possible influence of acarbose on GLP-1 release and gastric emptying in Type 2 diabetic patients after a mixed test meal.

PATIENTS AND METHODS

Ten Type 2 diabetic patients were tested with 100 mg acarbose or placebo served with a mixed meal that was labelled with 100 mg 13C-octanoic acid. Plasma concentrations of glucose, insulin, C-peptide, glucagon, GLP-1 and GIP were determined over 6 h. Gastric emptying was measured by determining breath 13CO2 using infrared absorptiometry. Statistics repeated-measures anova.

RESULTS

Gastric emptying rates (t1/2: 162 +/- 45 vs. 163 +/- 62 min, P = 0.65) and plasma concentrations (increasing from approximately 12 to approximately 25 pmol/l, P = 0.37) and integrated responses of GLP-1 (P = 0.37) were not changed significantly by acarbose treatment. Postprandial plasma glucose concentrations (P < 0.0001) and their integrated responses were lowered by acarbose (by 64%; P = 0.016). The plasma concentrations of insulin and C-peptide were reduced (P = 0.007 and 0.057, respectively) by acarbose, while glucagon was not changed (P = 0.96). GIP plasma concentrations (increasing with placebo from approximately 10 to approximately 85 pmol/l and with acarbose to approximately 55 pmol/l (P < 0.0001) and their integrated responses were significantly lowered (by 43%) by acarbose (P = 0.021). After 2 weeks of acarbose treatment (50 mg t.i.d. for the first and 100 mg t.i.d. for the second week, n = 6), similar results were found.

CONCLUSIONS

In hyperglycaemic Type 2 diabetic patients, ingestion of acarbose with a mixed test meal failed to enhance GLP-1 release and did not influence gastric emptying.

Glucagon-like peptide 1 (GLP-1) in the treatment of diabetes

Glucagon-like peptide 1 (GLP-1) was discovered as an incretin (insulinotropic gut) hormone. Biological actions of GLP-1 in healthy and type 2 diabetic subjects include (a) stimulation of insulin secretion in a glucose-dependent manner, (b) suppression of glucagon, (c) reduction in appetite and food intake, (d) deceleration of gastric emptying. In animal experiments, in addition, (e) stimulation of beta-cell neogenesis, growth and differentiation in animal and tissue culture experiments, and (f) in vitro inhibition of beta-cell apoptosis induced by different agents have been observed. Since the incretin effect--the higher insulin secretory response to oral as compared to intravenous glucose loads - is reduced in patients with Type 2 diabetes, GLP-1 has been used to pharmacologically replace incretin. Intravenous GLP-1 can normalise, and subcutaneous GLP-1 can significantly lower plasma glucose in the majority of patients with Type 2 diabetes. The magnitude of this effect does not greatly depend on patient characteristics such as age, sex, obesity, or baseline insulin and glucagon, with minor influences of previous antidiabetic therapy and actual metabolic control. GLP-1 itself, however, is inactivated rapidly in vivo by the protease DPP IV and can only be used for short-term metabolic control, such as in intensive care units (potentially useful in patients with acute myocardial infarction, coronary surgery, cerebrovascular events, septicaemia, during the perioperative period and while on parenteral nutrition). For more long-term metabolic control, incretin mimetics (agonists at the GLP-1 receptor) with more favourable pharmacokinetic profiles should be used.